Packing sheets are produced on special calenders which have a large diameter cylinder heated by a heating medium and an unheated, usually cooled, pressure cylinder of smaller diameter. The large diameter of the heated cylinder is necessary in order to produce packing sheets economically. For the packing sheets are produced in a discontinuous process in which the previously mixed mixture of rubber material, fibers and solvent is fed into the gap between the cylinders and is built up in thin layers on the heated cylinder during many revolutions thereof. The layer of material thus built up on the heated cylinder is vulcanized by the heat of the heated cylinder and forms a hollow, cylindrical packing sheet. The diameter of the heated cylinder is, for example, 1300 mm or more and the length of the heated cylinder is, for example, 2000 mm. After completion of the vulcanization, the heated cylinder and the pressure cylinder are stopped and the layer of material on the heated cylinder is cut by a cut parallel to the axis of the heated cylinder to form a sheet whereupon one cut edge of the sheet is gripped and the sheet is drawn from the heated cylinder.
The previously used mixture of rubber material, asbestos fiber and a solvent can no longer be used. The substitute fibers presently available for the production of packing sheets are considerably more difficult to handle. The process parameters with respect to temperature and rate of rotation have become very critical. If they are not observed, defective products are produced. While previously packing sheets using asbestos fiber could be produced with the technical knowledge and ability of the machine operator, with the fibers used today and the constantly increasing technical requirements, the programming of the machine has become more complicated and requires the use of a microprocessor. Contributing to these difficulties is the fact that the mixture for the production of packing sheets has strong heat insulation properties so that during the build-up of the packing sheet, the heat insulation becomes greater as the sheet becomes thicker, with the result that the temperature of the outer surface of the sheet progressively decreases whereby vulcanization is retarded and becomes non-uniform. In order to obtain sufficient vulcanization of the outer layers, the duration of the work process and number of revolutions of the heated cylinder must be increased, which makes the process uneconomical. The solution of these problems places very high demands on a suitable process procedure. However, these high demands are not the only objection. The temperature decrease of the outer layer as the sheet becomes thicker affects the quality of the packing sheet produced.